When standardized cat extracts were licensed, they were arbitrarily assigned 100,000 Allergy Units (AU)/ml if they contained 10-19.9 Fel d I u/ml. With the introduction of the ID50EAL method, biological Allergy Units are currently assigned based on skin test potency determined in highly skin reactive cat allergic subjects. Two standardized cat extracts (standardized cat pelt and standardized cat hair extracts) widely differing in composition of non-Fel d I allergens, but containing equal Fel d I contents of 10 and 20 Fel d I u/ml were assayed to estimate the number of Bioequivalent Allergy Units (BAU) to be assigned to each. Both the 10 and 20 Fel d I containing extracts had mean D50s' between 11 and 12.9 and therefore merit assignment of 10,000 BAU/ml rather than the 100,000 AU/ml arbitrarily assigned. Manufacturers have been notified of this change in labeling.